Alloy steel



Patented Oct. 6, 1953 ALLOY STEEL Orin W. McMullan, Belleville, Mich., assignor to Bower Roller Bearing Company, Detroit, Mich., a corporation of Michigan No Drawing. Application May 14, 1951, Serial No. 226,285

2 Claims.

This invention relates to metallurgy and, in particular, to ferrous metal alloys.

One object of this invention is to provide an alloy steel which will maintain its hardness and strength and resist wear, distortion and permanent size changes while operating at or being subjected to higher temperatures than are normally encountered in moving steel parts or those in contact with moving parts.

Another object is to provide such an alloy steel which will contain ingredients that are readily available and least apt to be critically scarce in time of war or national emergency.

Another object is to provide such an alloy steel which may be heat-treated at substantially normal temperatures and with ordinary heat-treating equipment rather than by the expensive and rapidly deteriorating equipment at the high temperatures required for heat-treating high speed and other high alloy steels that possess high hardness at elevated temperatures.

Another object is to provide an alloy steel which may be easily and rapidly carburized to give the surface a high carbon content and after heat treatment, a high surface hardness which will render it especially suitable for use in locations requiring a high order of wear-resistance, such as in bearings and especially anti-friction bearings, the alloy being capable of absorbing carbon readily from the gas, liquid or solid compounds ordinarily used in carburizing steel.

Another object is to provide such an alloy steel, as set forth in the preceding objects, which can be readily processed, such as by drilling, turning, grinding, shaping, milling, broaching and so forth.

This is a continuation-in-part of my co-pending application Serial No. 69,049 filed January 3, 1949, for Alloy Steel, now abandoned.

The ordinary carbon and alloy steels used in machine parts in contact under rolling or sliding movement, such as anti-friction bearings, lose their high hardness if subjected to excessive temperatures, that is, above 300 or 350 F. normally used for drawing or tempering such parts. Temperatures above the previous draw temperature temper or soften the parts so that they gall or seize in operation. Certain alloy steels do resist such softening, some as high as 1000 to1200 F., but for reasons stated below they are used chiefly for tools and dies.

The essential alloying elements added to carbon steel to maintain high hardness or produce secondary hardness (an increase in hardness when tempering temperature is increased), are

the carbide forming elements tungsten and mo lybdenum used along with other carbide formers, chiefly chromium and vanadium. The complex alloy carbides formed by these elements require high heat-treating temperatures, frequently 2000" to 2i00 F., to put them into solution and to develop secondary hardening power. The special furnaces and equipment required are expensive to maintain-factors which handicap these steels in a highly competitive market. Nickel is ordinarily not used, or is only occasionally used and then only to a limited amount, in such steels since it tends to retain, after quenching, an excessive amount of austenite, re sulting in a structure with low indentation hardness as measured by the Brinell or Rockwell test. Although austenitic steels are file hard, they are subject to plastice flow and change of shape under pressure.

A high carbon content is necessary to develop high hardness. Hardness throughout may be desired for some purposes, but for other applications merely a high surface hardness is sufficient or may even be desirable. The carbide forming elements previously mentioned belong to the so-called gamma loop closing group, or those which either prevent the absorption of carbon while the steel is in the solid condition or raise the temperature of its absorption beyond that possible to use in ordinary production car burizing furnaces. .As such steels must be made by adding the carbon to the molten bath, they have a high carbon content all the way through, as originally made. High carbon, together with high alloy content, however, makes machining difiicult and such steels are not easily processed by drilling, turning, shaping or the like and, indeed, require long, expensive annealing treatments to make these operations possible.

The alloy steel of the present invention is designed to eliminate these difiiculties and provide a more practical alloy having the advantages set forth in the foregoing objects. The composition developed is the, result of extensive research into the efiects of the individual elements and the effect of combinations of two or more in numerous experimental melts .of steel, as determined by tests on samples thereof.

The most suitable steel for attaining the objects of the present invention has been determined as one containing molybdenum, nickel and chromium as alloying elements. Molybdenum is an efiective carbide forming element imparting secondary hardness or'high hardness after tempering up to 1000 F, or higher, and at the same time it is the most readily available of such elements in time of national emergency. Nickel is also an essential element in this steel. Nickel is an austenite forming element and increases carbon or carbide solubility at lower temperatures, offsetting the reverse effect of molybdenum and chromium. Thus, the addition of "nickelniahes a higher alloy steel that can be carburized and heat-treated at normal temperatures in ordinary production furnaces. The possibility of carburizing, in turn, permits the use of low carbon steels that can be more readily annealed, machined,

cold formed, etc. These case "hardened steels have the same advantages of the lower alloy case hardened steels, that is, a high carbon content, hard and wear resistant surfaceandateughfcbre. However, a steel made with high carbon content will possess the temperature resisting properties after suitable heat treatment, and may be used if processing difficulties are not too much of a handicap. Molybdenum and nickel insui'table ambufnjts will produce-a steel of-good characteristics, but it has'been found'th a't'th e additionof chromiu'mincreases the har ness at anyjiiven temperature by a few points in Rockwell C hardness. cr mmium also can be used to replacesbnie of the molybdenum otherwise required, I an advantage because of the greater 'cost ofthelatter. I

The preferred composition of the non ferrous ingredients of the alloy steel of this invention-and the preferable range, as expressed as percentages of the total alloy (the remainder being substantiallyall iron) are as follows:

Composition mesa:

Nickel .I

s a'es 'e s wl massage? E 999.

in the usual production melting furnaces for.

higher alloy content st'eels which, because 'Of certaincomp'onents of their alloyci'jntent, particularly chromium, are ordinarily electrically heated are or induction furnaces. The Steelis p'rferabl-y made according to 'finegrained pr'aetiee to permit direct quenching from carburizi'n'g temperature, although reheating after ca'rbu'rizihg-inafy be employed. Carburizi'ngis done by ineanshf 'a nyof the commercial carburizing co'fnpounds, gaseous, liquid orsolid, within the usuahcominercial range of 1650 to =I800 F, with'the latter preferredfespecially when thealloy content is toward the high side'of the range. The-steel'carburizes nearly as rapidly as the commercial carburizin'g steels of lower alloycontent. The-actual time depends on the depth of the-case (orsurface layer) required by the design of the part. Parts arecooledfrom the carburizing temperature, or after reheating to a similar range, at a rate to provide an austenite structure in the case or surface layer. This can 'bedone by oil quenching or cooling in air. Air "cooling in sizes up'to'Z" round has been employed. Very large sizes, however, would require an oil quench. The alloy content is balanced so that austenite will be retained when the steel is cooled, but not so stable that it will not be converted to martensite on tempering or require excessive time to-do so.

Because "of the plastic {f ow oi the eiustenite and the highly stable austenite produced as a result of the nickel content, the tempering procedure is an important part of this invention and difiers train that usually employed. According to the present invention, austenite is converted to mar- ;tensiite or other 'cen'stituents of high Rockwell hardness by multiple treatments in the range of 600 to 120091 One treatment may result in high names, but requires an excessively long time, espe'ciauy with the alloy content at the high limitsgand leaves the martensite in an untempered, highly stressed, brittle condition. To avoid this at least two and preferably several tempering treatments 'are r'equiied. Less total tune is required by several'shprttreatments than byfewr longer treatments. B'etw'e'entreatnierits the steel is cooled 'to approiiimaely room temperaturehm the time at room temperature is -not critical. By a choice 6f temperingtemperaiureassume at heat, any desired per cent ofthe austenite can be decomposed in one'treatin'ent. The heat treatment tempers the mart'e'ns'ite formed, ence the stresses producedand the dangei ofcracking can be precisely controlled. 'Thi's 'f'ea'tfifent alsb'p'foduces a part relatively free "aistortieii a shape,

Parts treated in this f'm'an' hergaebordiiig "to 'the a present invention, with'five'teiiip'ring treatments of tWO hours 68.01] at 1000" F. 'will hall/e 9) fih el hardness in the range 6f 55 to 65 Rockwell C. hardness, depending onthe aetuaianaiysis, within the range Of this invention. Further rpetrtiOIlS 01 Very long holds at I000 Ffwi'll Tprodliith only a minor 'drop in hardness in the order or 1 170 5 points Rockwell C. Paffts tni'pe fe d five 2 hour periods at '10'0'0' F. are very stable in hardness when reheated to lower temperatures, forexarn'ple, l'pa'fts BURc remained unchanged when given single "treatments up to 30 hours or repeated 'itreatmeias totaling hours at 900 F. The struetur'eo'i thebase or surface layer, as quenched, is austeiiiteana carbide. After tempering this becomes carbide and inarte'n's'ite or other hard decomposition products. For maximum stability of hardness and size over extended periods of time, austeni'te is completelydecomposed hythe tempering treatments. If the operating temperature of the part is considerably below the tempering't'ernperature employed, complete 'aeeompos'iuon of the austenite is not necessary, but the major portion must be "decomposed to develop h igh Rockwell hardness andprevent plastic flow under pressure.

In "producing articles rrbm the alloy steel or my invention, it is necessary to carburi-ze'and quench the articles before carrying out the multiple tem ering 'csperatidris, in order to produce the; requisite hardness 'at the tempering temperature of less than f) F. Without such carbu'rizin'g, quenching "and multiple tempering, neither the alloy steel ef' -iny'inve'ntion nor any alloy steel of low carbon contentknown to me will achieve the desired hardness and resistance to wear at the above mentioned tempering temperature of less than 1260? "on the contrary, such steels without such carburization are relatively soft and wear away quickly'when usedfor nus.

articles such as bearings, particularly when these bearings are subjected to high operating temperatures.

The hardness of the alloy steel of the present invention is not developed by quenching but, on the contrary, is developed during multiple tempering, the secondary martensite so formed being more resistant to tempering and softening during operation than is the primary martensite formed on quenching. The alloy steel of the present invention in employing carburizing, quenching and multiple tempering, utilizes nickel to lower the temperatures of carburizing and hardening in order to retain more austenite so that more secondary martensite may be produced.

Numerous samples of alloy steels have been prepared under my direction in accordance with this invention and tested as regards their hardness under high temperature operating conditions and as to their other physical, chemical and metallurgical characteristics, and I have also prepared actual bearings that have been tested for performance characteristics in engines. Actual samples of alloy steel made according to the present invention, before being subjected to such multiple tempering, possess a hardness of only 30 Rockwell C, whereas during such heat treatment, the hardness rises to 60 Rockwell C.

Alloying elements such as chromium, molybdenum, tungsten and vanadium found in conventional tool steels, raise the so-called gamma loop, prevent carbon absorption at lower temperatures, require high temperatures to dissolve the carbides present, and retain less austenite when quenched, especially from lower temperatures than obtainable with a nickel content steel. For example, a steel containing 0.13% carbon, 3.15% chromium, 5.00% molybdenum, and 1.18% vanadium when carburized at 1900 F. was found upon test to possess a shallow spotty case of low carbon content and retained little austenite when direct-quenched, in contrast to the alloy steel of the present invention which carburizes at normal temperatures around 1700 F. and is highly austenitic when direct-quenched in oil or air cooled.

The alloy steel articles of the present invention can be produced at less than one-third of the cost per pound of so-called high-speed steel. Moreover, the annealing cost and machining cost of such articles made from the softer low carbon steels of the present invention before being subjected to the subsequent carburization and multiple tempering, are less than those made from high carbon steel, yet such articles are rendered equally wear-resistant by subsequent carburizing and multiple tempering. Furthermore, the cost of finished parts made of the alloy steel of the present invention is further lowered by the use of lower quenching temperatures than are ordinarily employed, as well as by the use of ordinary production furnaces rather than the expensive high temperature furnaces such as are used up to 2400 F. for high speed steel.

In view of the critical effect of the percentages of the ingredients of the alloy steel of the present invention and also in view of the critical eifect of carburizing and multiple tempering, the in-- vention cannot adequately be claimed by the percentage composition alone of its ingredients, but solely by the percentage composition in conjunction with process limitations involving carburizing, quenching and multiple tempering. Moreover, the percentage composition and ranges thereof as set forth above and in the appended claims are also critical because steels with percentage compositions outside these percentages do not possess the hardness characteristics upon multiple tempering, as set forth above, and do not possess the wear-resistant characteristics of the alloy steel of the present invention at high operating temperatures.

In particular, if the carbide-forming elements chromium and molybdenum are beyond the ranges set forth above and in the appended claims, excessively high heat-treating temperatures of over 2000 F. are necessary in order to render such steels austenitic as quenched, and if the nickel content is also above the ranges herein set forth, in addition to excessive percentages of the carbide-forming elements chromium and molybdenum, then the austenite formed by quenching becomes so stable that its transformation into martensite by tempering to reach the desired hardness becomes either impossible or at least commercially infeasible.

What I claim is:

1. An alloy steel article possessing, at high operating temperatures, high hardness and resistance to wear such as required by parts in contact under rolling or sliding movement, which article consists essentially of approximately 3.00% nickel, approximately 1.50% chromium, approximately 5.00% molybdenum, approximately 0.15% carbon, approximately 50% manganese, approxi mately 0.30% silicon, and the remainder substantially all iron, and which has been carburized, quenched and multiple-tempered to increase its hardness by repeatedly heating it to a temperature between 600 F. and 1200 F. and cooling it to room temperature between such repeated heatmgs.

2. An alloy steel article possessing, at high operating temperatures, high hardness and resistance to wear such as required by parts in contact under rolling or sliding movement, which article consists essentially of 2.50% to 3.00% nickel, 1.00% to 3.00% chromium, 2.00% to 6.00% molybdenum, 0.05% to 0.30% carbon, 0.20% to 1.00% manganese, 0.1% to 0.50% silicon, and the remainder substantially all iron, and which has been carburized, quenched and multiple tempered to increase its hardness by repeatedly heating it to a temperature between 600 F. and 1200 F. and cooling it to room temperature between such repeated heatings.

'ORIN W. MCMULLAN.

References Cited in the file of this patent Gill. Published in 1944 by the American Society for Metals, Cleveland, Ohio.

Steel Processing, November 1944, pages 721 

2. AN ALLOY STEEL ARTICLE POSSESSING, AT HIGH OPERATING TEMPERATURES, HIGH HARDNESS AND RESISTANCE TO WEAR SUCH AS REQUIRED BY PARTS IN CONTACT UNDER ROLLING OR SLIDING MOVEMENT, WHICH ARTICLE CONSISTS ESSENTIALLY OF 2.50% TO 3.00% NICKEL, 1.00% TO 3.00% CHROMIUM, 2.00% TO 6.00% MOLYBDENUM, 0.05% TO 0.30% CARBON, 0.20% TO 1.00% MANGANESE, 0.1% TO 0.50% SILICON, AND THE REMAINDER SUBSTANTIALLY ALL IRON, AND WHICH HAS BEEN CARBURIZED, QUENCHED AND MULTIPLE TEMPERED TO INCREASE ITS HARDNESS BY REPEATEDLY HEATING IT TO A TEMPERATURE BETWEEN 600* F, AND 1200* F. AND COOLING IT TO ROOM TEMPERATURE BETWEEN SUCH REPEATED HEATINGS. 